The main idea here is much like any other gambit: Sacrificing material for a tempo and a lead in development. In this line, even the declined variations tend to let white tear open the center and start a vicious attack.

This is particularly fun against the French Defense, since French players tend to like slow and/or closed positional games, and are usually unhappy to see the center gone in the first few moves. Ripping open the center leads to a fast-paced tactical game. This is not an opening for the faint of heart.

The accepted variation (3. ... dxe4 4. f3 exf3 5. Nxf3 ...) leads white to a violent kingside attack, many times crushing the opposing king before black can get all of his pieces in order. If you're a strong attacking player who's not afraid of storming the enemy king in his home, you should consider adding the DDG to your repetoire.